The present invention relates to a distributed feedback semiconductor laser (hereinafter referred to simply as a "DFB laser") which is provided with a diffraction grating having a phase shift corresponding to a quarter wavelength of light so as to implement a stable singlewave-length operation.
The DFB laser is now being developed as a light source for low-loss optical fiber communications because it incorporates a diffraction grating of excellent wavelength selectivity and is able to perform a single-wavelength operation with ease. In particular, the DFB laser of the type in which the phase of the diffraction grating is shifted at the center portion thereof by a quarter of the wavelength of light is excellent in the selectivity of a single wavelength, and it is also expected that its production yield will be increased. Conventional phase-shift DFB laser are excellent in the selectivity of single wavelength but have an inefficient structure in terms of laser efficiency.